Page 43 - Handbook of Structural Steel Connection Design and Details

Fasteners and Welds for Structural Connections

28 Chapter One

With too rapid cooling, cracks might form in a weld. Possible causes
are shrinkage of weld and heat-affected zone, austenite-martensite
transformation, and entrapped hydrogen. Preheating the base metal can
eliminate the first two causes. Preheating reduces the temperature gra-
dient between weld and adjacent base metal, thus decreasing the cool-
ing rate and resulting stresses. Also, if hydrogen is present, preheating
allows more time for this gas to escape. Use of low-hydrogen electrodes,
with suitable moisture control, is also advantageous in controlling hydro-
gen content.
High cooling rates occur at arc strikes that do not deposit weld metal.
Hence strikes outside the area of permanent welds should be avoided.
Cracks or blemishes resulting from arc strikes should be ground to a
smooth contour and checked for soundness.
To avoid cracks and for other reasons, AWS specifications require
that under certain conditions, before a weld is made the base metal
must be preheated. Table 1.10 lists typical preheat and interpass

TABLE 1.10 Requirements of AWS D1.1 for Minimum Preheat and Interpass
Temperatures, °F, for Welds in Buildings for Some Commonly Used Structural Steels*

Shielded Shielded
metal-arc metal-arc;
with low- submerged-arc,
hydrogen gas metal-arc, or
electrodes; flux-cored arc with
submerged- electrodes or
arc, gas electrode-flux com-
metal-arc, or Shielded bination capable of
flux-cored metal-arc depositing weld
arc with low- metal with a
Shielded hydrogen maximum
metal-arc ASTM A36, electrodes; diffusible
with other A53 grade submerged- hydrogen content
than low- B, A441, arc, gas of 8 mL/100 g
hydrogen A501, A529 metal-arc, or when tested in
electrodes grades 50 flux-cored accordance with
Thickness and 55, arc AWS A4.3
at thickest ASTM A36, A572 grades
part at A53 grade 42, 50, and ASTM A572 ASTM A913 ‡
point of B, A501, 55, A588, grade 60 grades 50, 60,
welding, in A529 A992 and 65 and 65
3 32 † 32 † 50 32 †
To ⁄4
3 1 150 50 150 32 †
Over ⁄4 to 1 ⁄2
1 1 225 150 225 32 †
1 ⁄2 to 2 ⁄2
1 300 225 300 32 †
Over 2 ⁄2
*In joints involving different base metals, preheat as specified for higher-strength base metal.
† When the base-metal temperature is below 32°F, the base metal shall be preheated to at least
70°F and the minimum interpass temperature shall be maintained during welding.
‡ The heat input limitations of AWS D1.1 paragraph 5.7 shall not apply to A913.

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